The present invention relates generally to star trackers, and more particularly, to a system for stellar inertial attitude determination.
Satellites and other spacecraft are in widespread use for various purposes including scientific research and communications. Many scientific and communications missions, however, cannot be accurately fulfilled without consistently monitoring and controlling the 3-axis attitude of the spacecraft. In many applications, the satellite must be positioned to direct communication signals in particular directions or to receive signals from specifically located sources. Without accurate control over spacecraft 3-axis attitude, the transmission of such signals is hindered and at times impossible.
Many modern spacecraft use star trackers for 3-axis attitude control. The systems for 3-axis attitude determination generally include one or more star trackers and a separate 3-axis orthogonal gyroscope. During normal operation, star trackers provide continuous attitude information and the 3-axis orthogonal gyroscope is needed to provide rate information and is constantly corrected by the star trackers.
Star trackers are capable of providing 3-axis attitude information by using a star pattern seen in the field of view (FOV) of the star tracker. Unfortunately, the star pattern selected by these star trackers is not always the most optimal star pattern available. In fact, the current art of star selection within the real time tracker FOV is somewhat arbitrary with regard to improving overall system accuracy (e.g., it is commonly suggested that the first few brightest stars be selected).
To improve spacecraft attitude control performance, stellar inertial attitude determination algorithms employ a carefully designed star catalog. The current known art in star selection for star trackers mainly concentrates on generating star catalogs with certain properties. Typically, a star catalog is xe2x80x9cequalizedxe2x80x9d by eliminating stars in the dense areas of the star catalog. The resulting star catalog becomes approximately uniformly distributed and is known to improve stellar inertial attitude determination performance. However, star catalog equalization only assures equal distribution in an average sense and may fail to guarantee desirable star patterns in real time for a moving tracker field of view (FOV). Moreover, a fixed star catalog does not allow the tracker to actively and dynamically select star patterns in order to optimize the stellar inertial attitude determination performance. In addition, the star catalog must be reloaded once the spacecraft condition and star tracker condition are changed.
Therefore, there exists a need to improve stellar inertial attitude determination performance by actively and dynamically selecting star patterns from stars visible to the star tracker in real time on-board the spacecraft.
It is, therefore, an object of the invention to provide an improved and reliable system for stellar inertial attitude determination. Another object of the invention is to provide a system capable of actively and dynamically selecting star patterns.
In one embodiment of the invention, an apparatus for determining stellar inertial attitude based upon a plurality of stars includes a star tracker and a controller. The star tracker is used to sense the positions of stars and generate signals corresponding to the positions of the stars as seen in its field of view. The controller checks a star catalog and actively determines which stars to track. The controller does this determination in terms of each star""s relative geometry. The controller then commands the star tracker to track these stars and uses them to determine the spacecraft attitude.
The present invention thus achieves an improved system for stellar inertial attitude determination. The present invention is advantageous in that many types of measurement noises for multi-star tracker systems can be minimized.
Additional advantages and features of the present invention will become apparent from the description that follows, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.